lördag 29 oktober 2011

Reality and Fiction of Stefan-Boltzmann's Radiation Law

Human accountant in charge of a non-physical fictional Stefan-Boltzmann Law.

In previous posts on radiative heat transfer I have compared two different formulations of Stefan-Boltzmann's Radiation Law (SB) for the radiative exchange of heat energy between two blackbodies of different temperatures:

one-way transfer from hot to cold,

two-way transfer with net transfer from hot to cold.

To see which formulation best represents physics, recall the wave equation model of a blackbody as a vibrating string with displacement $U$ subject to radiative damping:

$U_{tt} - U_{xx} = f - (-\gamma U_{ttt})= f + \gamma U_{ttt}$,

which expresses a balance between the string force $U_{tt} - U_{xx}$ and the net force $f+\gamma U_{ttt}$ from the radiation pressure $-\gamma U_{ttt}$ and the exterior forcing $f$. For details see Mathematical Physics of Blackbody Radiation.

The essential aspect is now the interplay between the internal energy (density) $IE$ of the vibrating string

$IE=\frac{1}{2}(U_t^2 + U_x^2)$

and the net forcing $f +\gamma U_{ttt}$, which is expressed in the following energy balance obtained by multiplying the force balance by $U_t$ and integrating in space and time to get

$\int \frac{dIE}{dt}dxdt = \int (f +\gamma U_{ttt})U_tdxdt$.

We se that the rate of change $\frac{dIE}{dt}$ of internal energy $IE$ is balanced by a net force $f + \gamma U_{ttt}$ scaled with $U_t$. We can interpret $E=\int IE\, dx$ as an accumulator recording the net effect of the forcing and radiation, with $E$ proportional to $T_U^2$ with $T_U$ the temperature of the blackbody (with displacement) $U$.

In the case the forcing $f$ is delivered by another blackbody with displacement $V$ and temperature $T_V> T_U$, the energy balance takes the form

This is the version of Stefan-Boltzmann's Radiation Law (SB) cherished in climate science describing the heating $\frac{dE}{dt}$ as the difference of two gross flows of incoming radiation $\sigma T_V^2$ and outgoing radiation $\sigma T_U^4$.